(57) Reproduceability and reliability problems associated with using a solder preform to bond a semiconductor laser chip (1) to a heat sink (2) are avoided by a diffusion assisted bonding method. Both surfaces to be bonded are coated with relatively thick layers of gold (3,7), one of which is then coated with a submicron thickness tin layer (5) whose surface is protected from oxidation with a further coating (6) of gold. The two components are assembled and heated to bond them together by causing a molten gold-tin eutectic layer to be formed which then diffuses into the gold layers without penetrating right through them.